Added new role for moderator to Groups

• Added:
- new role for moderator to Groups

app/serializers/rest/group_relationship_serializer.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class REST::GroupRelationshipSerializer < ActiveModel::Serializer
-  attributes :id, :member, :admin
+  attributes :id, :member, :admin, :moderator
 
   def id
     object.id.to_s
@@ -15,4 +15,8 @@ class REST::GroupRelationshipSerializer < ActiveModel::Serializer
     instance_options[:relationships].admin[object.id] ? true : false
   end
 
+  def moderator
+    instance_options[:relationships].moderator[object.id] ? true : false
+  end
+
 end